The delivery of drugs to a patient is conventionally performed in a variety of different ways. For example, intravenous delivery is by injection directly into a blood vessel; intraperitoneal delivery is by injection into the peritoneum; subcutaneous delivery is under the skin; intramuscular delivery is into a muscle; and oral delivery is through the mouth. One of the easiest methods for drug delivery, and for collection of body fluids, is through the skin. Skin is composed of the epidermis, including the stratum corneum, the stratum granulosum, the stratum spinosum, and the stratum basale, and the dermis, containing, among other things, the capillary layer. The stratum corneum is a tough, scaly layer made of dead cell tissue that extends around 10-20 microns from the skin surface and has no blood supply. Because of the density of this layer of cells, moving compounds across the skin, either into or out of the body, can be very difficult.
Current techniques for delivering local pharmaceuticals through the skin include methods that use needles or other skin piercing devices and methods that do not use such devices. Those methods that do not use needles typically involve: (a) topical applications, (b) iontophoresis, (c) electroporation, (d) laser perforation or alteration, (e) carriers or vehicles, which are compounds that modify the chemical properties of either the stratum corneum and/or the pharmaceutical, (f) physical pretreatment of the skin, such as abrasion of the stratum corneum (e.g., repeatedly applying and removing adhesive tape), and (g) sonophoresis, which involves modifying the barrier function of stratum corneum by ultrasound. Invasive procedures, such as use of needles or lances, can effectively overcome the barrier function of the stratum corneum. However, these methods suffer from several major disadvantages, including pain, local skin damage, bleeding, risk of infection at the injection site, and creation of contaminated needles or lances. These methods also usually require a trained administrator and are not suitable for repeated, long-term, or controlled use. Additionally, drug delivery through the skin has been relatively imprecise in both location and dosage of the pharmaceutical. Some of the problems include movement of the patient during administration, delivery of incomplete dosages, difficulties in administering more than one pharmaceutical at the same time, and difficulties in delivering a pharmaceutical to the appropriate part of the skin. Drugs have traditionally been diluted to enable handling of the proper dosages. This dilution step can cause storage as well as delivery problems. Thus, it would be advantageous to be able to use small, precise volumes of pharmaceuticals for quick, as well as long-term, delivery into and/or through the skin.
Microneedles have been proposed for this purpose. The microneedles typically have a hollow shaft, similar to larger conventional medical needles, so that drug formulations may be delivered through the hollow shaft. Various mechanisms have been employed to initiate the flow of the drug formulation through such devices. U.S. Pat. No. 6,611,707 to Prausnitz et al. and U.S. Pat. No. 5,527,288 to Gross et al., for example, describe devices that each include a drug reservoir positioned over a housing that includes an array of hollow microneedles. A drug formulation is delivered from the reservoir by applying a force against the drug itself or against the reservoir, such as by pressing against the top of a flexible reservoir bag, to cause the formulation to flow out through the microneedles. Unfortunately, the flow rate of the drug formulation injected into the skin using such force is often far greater than the absorption rate of the skin itself. As a result, all or a significant portion of the drug formulation will often flow upwards at the interface between the skin and the microneedles to the surface of the skin.
As such, a need currently exists for a drug delivery system that can effectively deliver a drug formulation in a manner to improve the controlled delivery and bioavailability of the drug.